It is well known that torque spikes can be produced by an engine during the start-up and at shut-down procedures. Typically, such torque spikes are the result of the fact that engines, and the mechanism of the output drive connection, often have a resonant frequency which occurs at some engine speed that is less than idle speed. Hence, as the rotational speed of the engine passes through that speed which produces torque resonance during its start-up and/or shut-down procedures, the torque supplied by the power output shaft of the engine to the downstream components of the transmission may be greater than would otherwise be expected. This increased torque output spikes during the power stroke of the engine.
In general terms the power available at the output shaft of an engine may be considered as tire product of tire output shaft speed multiplied by the torque delivered by the output shaft. Engine operating speeds per se, and even wide variations of that speed, do not impart deleterious forces to the downstream components of the transmission. Torque spikes, on the other hand, can impart deleterious forces to the components of the transmission at virtually any rotational speed.
A recent trend among the manufacturers of engines has increased the incidence of deleterious torque spikes. Specifically, some manufacturers have elected to reduce the number of pistons in order to reduce the cost of building engines adapted to supply a given horsepower. When two engines deliver the same rated horsepower at a given rotational speed, the engine having the lesser number of cylinders, and therefore a lesser number of power strokes per revolution, will deliver a greater torque during each power stroke. As such, the torque applied by such an engine, when rotating at the resonant frequency, can have a significantly deleterious effect on the downstream components of the transmission. And yet, when an engine is operating at speeds above those which impart the resonant frequency, it is counterproductive to reduce the torque available to the transmission from each power stroke of the engine.